N-13 Ammonia Myocardial Uptake but not Blood Flow Identifies Regions With Positive Contractile Reserve to Low-Dose Dobutamine: In patients with chronic coronary artery disease, the relation between regional myocardial blood flow and regional contractile response to low-dose dobutamine is complex. Recently we showed that beyond its value as a myocardial blood flow tracer, late N-13 ammonia uptake (metabolic trapping) provides useful information regarding functional recovery after revascularization. In the present study, we determined whether late N-13 ammonia uptake may identify regional contractile response to low-dose dobutamine. Twenty-nine patients with chronic coronary artery disease and left ventricular dysfunction underwent rest and low-dose dobutamine N-13 ammonia PET and cardiac MRI. Absolute myocardial blood flow was measured from the first 3 min after N-13 ammonia injection, using 2-compartment model, while late N-13 ammonia uptake was measured from the last 10-15 min of image acquisition. Myocardial blood flow did not differentiate asynergic regions with positive from negative contractile reserve with low-dose dobutamine but late N-13 ammonia uptake did. Thus, improvement in contractile function with low-dose dobutamine cannot be explained by the potential of that region to augment myocardial blood flow. These data suggest that improved contractile response to catecholamine is, in part, dependent on the metabolic status of the myocardium. - ammonia, myocardial blood flow (MBF), low dose dobutamine (LDD) - Human Subjects